Abstract
Different techniques have been combined to identify the structure and the chemical composition of siliceous breccia from a drill core of nickel laterites in New Caledonia (Tiebaghi mine). XRD analyses show quartz as a major phase. Micro-Raman spectroscopy confirmed the presence of reddish microcrystalline quartz as a major phase with inclusion of microparticles of iron oxides and oxyhydroxide. Lithoclasts present in breccia are composed of lizardite, chrysotile, forsterite, hedenbergite and saponite. The veins cutting through the breccia are filled with Ni-bearing talc. Furthermore, for the first time, we discovered the presence of diamond microcrystals accompanied by moissanite polytypes (SiC), chromite (FeCr2O4) and uranophane crystals (Ca(UO2)2(SiO3OH)2.5(H2O)) and lonsdaleite (2H-[C-C]) in the porosities of the breccia. The origin of SiC and diamond polytypes are attributed to ultrahigh-pressure crystallization in the lower mantle. The SiC and diamond polytypes are inherited from serpentinized peridotites having experienced interaction with a boninitic melt. Serpentinization, then weathering of the peridotites into saprolite, did not affect the resistant SiC polytypes, diamond and lonsdaleite. During karstification and brecciation, silica rich aqueous solutions partly digested the saprolite. Again, the SiC polymorph represent stable relicts from this dissolution process being deposited in breccia pores. Uranophane is a neoformed phase having crystallized from the silica rich aqueous solutions. Our study highlights the need of combining chemical and mineralogical analytical technologies to acquire the most comprehensive information on samples, as well as the value of Raman spectroscopy in characterizing structural properties of porous materials.
Highlights
The occurrences of moissanite (SiC) associated with NiMnCo, FeSi and FeC alloys, are described from many peridotites and podiform chromitites belonging to ophiolites in Turkey, China, Myanmar, Russia, Mexico and Cuba [1,2,3,4,5]
We report the first occurrence of diamond, lonsdaleite, moissanite among other reduced phases, and uranophane from silica rich rocks at the base of a nickel laterite profile, located in saprolite, at the Tiebaghi mine, New Caledonia
Before discussing the origin of diamond and the associated strongly reducing phases appearing in the pores of the breccia, we argue against that these phases are artefacts, generated during sample preparation through diamond sawing: (i) the sample preparation was carried out identically for the three other rocks, all sawn with the same diamond blade [22]. (ii) all the samples were studied by micro-Raman spectroscopy [22,23], but only the siliceous breccia hosts diamond polymorphs. (iii) the intergrowth of diamond and lonsdaleite is unlikely being a feature for diamond saw derived particles. (iv) clusters of chromite, diamond and SiC polymorph are observed in natural environment [2,5,28,31]
Summary
The occurrences of moissanite (SiC) associated with NiMnCo, FeSi and FeC alloys, are described from many peridotites and podiform chromitites belonging to ophiolites in Turkey, China, Myanmar, Russia, Mexico and Cuba [1,2,3,4,5]. It was found in meteorite, kimberlites [6], metasomatic rocks [7], peridotites, serpentinites and podiform chromitites [8,9]. The more frequent natural moissanite are the 3C, 4H, 6H and 15R polytypes, where C, H and R polytypes are cubic, hexagonal, and rhombohedral, respectively All these phases form at extremely low oxygen fugacity, 5 to 6 log units below the IW buffer [12]. A similar model was proposed for micro diamonds found in chromite in podiform chromitite in serpentinite from the Tehuitzingo ophiolite in South Mexico [16]
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